The invention relates to precious metal magnetic sputtering targets and the method of making the same. According to the invention, solid alloy powders manufactured via rapid solidification and elemental Pt are mechanically alloyed, densified, and machined into a sputtering target.
The objective of this invention is to achieve enhanced sputtering target characteristics from manufacturing and applications standpoints, through the utilization of innovative processing that enables novel microstructural design. The innovative process design has been developed with careful consideration of cost, lead time, and final product properties. The microstructural design has been developed with the intent to increase manufacturability and enhance product performance in application. In this invention, targets are manufactured using conventional processing steps such as gas atomization, powder mixing and milling, hot isostatic pressing, and machining, and, although the process steps themselves are not unique, the process steps have been strategically employed to achieve a superior sputtering target while maintaining competitive costs and lead-times in manufacturing. The novel microstructure created using the process described in this invention is characterized by a fine precipitate structure and a high degree of compositional homogeneity.
Gas atomization is a common method used to produce powdered metals for a broad range of industrial applications. It is generally recognized that this technique produces fine spherical powders with microstructures unique to rapidly solidified materials. Although atomization has been used in the sputtering target industry to make a range of alloy powders, atomization has not been used with the intent of reducing precipitate phase size in multiphase cobalt-based magnetic alloys. In this invention, gas atomization is used to produce alloy powders with fine microstructures, which lead to enhanced manufacturability during the mechanical working stage of the densified powders, and superior target microstructural and compositional homogeneity when compared to conventionally cast processing techniques. In general, the ductility of a multiphase metallic material is principally determined by the ductility of the continuous phase or phases in its microstructure. In a multiphase microstructure, the degree of continuity of a given phase is a function of its size and shape. For example, coarse microstructural features with high aspect ratios will become interconnected at much lower volume fractions than phases that are fine and spherical. This geometric fact can be summed up by stating that the percolation volume fraction limit of a given phase is inversely proportional to phase size and directly proportional to aspect ratio.
An example of this phenomenon occurs in CoCrPtB alloys containing greater than 6 atomic % B. When these materials are manufactured via conventional casting, they contain a brittle phase that is coarse and elongated. Because of its size and morphology, this phase is interconnected throughout the microstructure, and therefore dominates the mechanical behavior of the material and renders it brittle. In contrast, the same alloys, when manufactured in accordance with the invention tend to be much more ductile. This occurs because the brittle phases that are present in the microstructure are fine and equiaxed, and are therefore not continuous.
The microstructural differences that arise between the conventionally processed material, and the material processed using gas atomization are a result of the difference in the solidification rates of the two processes. Rapidly solidified materials tend to have much finer microstructures than conventionally cast materials. The intent of this invention is to strategically apply this phenomenon to the manufacture of cobalt-based sputtering targets, to promote superior mechanical working characteristics. The increases in ductility that are realized using the process steps outlined in the invention lead to high process yields during thermomechanical processing, which translate into manufacturing cost savings.
In addition, because the volume fraction of brittle phase in CoCrPtB alloys is a strong function of boron content, the relative increase in ductility, and therefore, the enhanced mechanical processing characteristics, for materials made in accordance with the invention becomes more pronounced as boron content increases. This amplifies the benefits of the invention as the requirements for increased boron content become more important in the media industry. Furthermore, in cases where the boron content of the finished target is above 10 at %, the invention becomes an enabling technology because conventional casting and mechanical working techniques become completely ineffective at these levels of boron.
Requirements for compositional homogeneity on thin film media have increased drastically over the last several years due to advances in head technology and disk storage capacity. This requirement has generated an industry need for multiphase sputtering targets with increased microstructural homogeneity, because increased target microstructure homogeneity reduces compositional gradients in sputtered films. In a paper by Harkness et al., J. Mater. Res., Vol. 15, No. 12, December 2000, p. 2811), this result is clearly demonstrated in the case of CoCrPtTa alloys. Although the alloy system investigated in this reference did not contain boron, and although the target microstructural manipulation processes did not involve the rapid solidification techniques discussed herein, the general results are salient to supporting the art described herein. The current invention employs two primary methods for attaining excellent compositional homogeneity within sputtering targets with complex chemistries. The first method is rapid solidification of the base master alloy powders used to make the targets. Rapid solidification leads to chemically homogeneous fine powders containing fine precipitates. The small scale of the particles and precipitates promote excellent point-to-point chemical uniformity within powder mixtures and, in turn, within finished targets. The second method is the mechanical alloying of the base powders using ball milling, or some other mechanical alloying technique. Mechanical alloying leads to alloy powder mixtures with extremely low chemical variability, and is therefore considered to be the optimum method for mixing powders of varying composition to create chemically homogeneous powder mixtures. The combination of these methods enables the fabrication of sputtering targets with greater chemical and microstructural uniformity when compared to targets made using conventional casting technology.
To demonstrate the degree of increased point-to-point homogeneity in sputtering targets manufactured via the invention relative to those made using conventional casting techniques, two targets were compared. Target 1 was made according to the invention and Target 2 was made using conventional techniques. Eight material samples were extracted from the targets in random locations using a standard drilling technique. Each sample was chemically analyzed for Co, Cr, Pt, B and Ta. The results indicates that the point-to-point chemical variability of Target 1 was significantly less than that of Target 2. The averages and standard deviations for the constituents measured in each of the samples is shown in the Table set forth below.
The standard processing paradigm in the sputtering target industry is to melt and cast alloys with the finished product composition. Platinum-containing alloys produced for sputtering target applications can be manufactured in a cost-competitive manner using standard alloying and casting practices. However, after examining the cost of manufacturing sputtering targets from Pt-containing atomized powders, it has become apparent that costs related to platinum losses during the atomization process render the process too costly to be a competitive sputtering target manufacturing process. Applicants have discovered a less costly method of manufacturing precious metal cobalt alloys by mechanically alloying elemental Pt powder with non-precious metal cobalt-based master alloy powders to reach the desired chemical composition, as opposed to atomizing a composition that contains Pt. Using this method, the Pt metal can be managed more efficiently during the entire target fabrication process, which substantially reduces the cost of manufacturing. This is a subtle, but critical, processing strategy employed in the invention to render it unique. To illustrate the potential for fabricated product cost savings, consider the following. A typical CoCrPtB alloy used in the data storage contains approximately 30% by weight Pt content. Gas atomization of the alloy, including Pt, results in a typical precious metal material yield loss of approximately 8%. In contrast, blending of Pt powder with gas atomized non-precious metal cobalt-based master alloy powders results in a typical precious metal yield loss of less than 2%. Pt currently costs over $20/gm (compared to Co at approximately $0.04/gm). Therefore, it is plain to see the tremendous cost benefit a 6% process improvement in Pt material yield provides. For example, this improvement in Pt yield loss translates to between a 20% to 30% fabrication cost reduction in CoCrPtB target products containing 12 atomic % Pt (approximately 30% by weight) and possessing a cylindrical geometry of approximate dimensions 7xe2x80x3 diameter by 0.300xe2x80x3 thick.
In addition to optimized cost structures, optimized process cycle time can offer a significant competitive advantage in the sputtering target industry. In the invention, significant cycle time reductions are achieved through using a set of master alloy powders with standardized compositions. These powders can be blended in various ratios to produce a wide range of final alloy compositions, thus avoiding the need to atomize a new batch of powder upon every newly requested alloy composition. The standard master alloy powders can be held in stock and the atomization cycle time can be eliminated from the process. Without this type of standardized material stocking method, it would be very difficult to have competitive cycle times on new material requests. In fact, the stocking of standard master alloy powders has been shown to reduce lead-times by as much as 80%.
An important quality metric that users of data storage targets consider when making purchasing decisions is lot-to-lot chemical consistency. The standard processing technique used to manufacture targets is a batch processes that comprises melting, casting and rolling steps. Typically, lot sizes weigh between 20 and 120 kg. A lot for which chemical composition is certified is defined as the product of the melting portion of the process. Lot-to-lot compositional variance due to natural process variations in standard melted CoCrPtB alloys are typically 0.5 atomic percent for each compositional species. Impurity levels also vary from lot-to-lot. In principle, to minimize chemical variation for a given alloy during the life of a media-manufacturing program, very large lots could be used, thus reducing the total number of lots necessary to manufacture the targets needed for the program. For example, if one very large lot were used, the lot-to-lot variation would be zero. However, maximum lot sizes for melt-processed materials are limited. Melting equipment with capacities greater than 150 kg are generally not economically feasible for manufacturing sputtering targets.
In the process of the invention, the blending portion of the process defines a lot for which chemical composition is certified. If, for example, ball milling is used for the blending portion of the process, it would be possible to economically produce large lots of material in one run. Ball mills with capacities in the 1000 kg range are not expensive relative to competitive manufacturing market metrics. The process of the invention allows the economic production of very large chemically homogeneous lots of material. This is a notable competitive advantage in the sputtering target industry.
The object of this invention is to provide for a multi-component, multiphase precious metal magnetic sputtering target typically comprising, but not limited to Co, Cr, Pt and B. To accomplish the object described above according to the present invention, there is provided a method for preparing the target, which comprises mechanically alloying a mixture of rapidly solidified cobalt master alloy powders with elemental platinum powder, and other elemental, ceramic, or alloy powders as required by the desired final composition. These and other objects and characteristics of the present invention will become apparent from the further disclosure of the invention that is given hereinafter with reference to the accompanying figures.